Theory of Operation 4
Keysight E4416A/E4417A Power Meters Service Guide 97
Ancillary Functions
The measurement assembly also measures the sensor resistor voltage to detect
whether a power sensor is present. If a power sensor is connected (or
disconnected) an interrupt to the A2 processor assembly is generated.
Circuitry on the board is then used by the processor to determine the sensor
model, and whether it's connected to the front or rear panel. The processor then
configures the board's ancillary functions, such as chop frequency, chop voltage
levels and also programmes the DSP with the appropriate code for that sensor
type. If the sensor was one of the E-Series then the processor reads calibration
data from the sensor's E2PROM using the I2C bus. The processor then uses the
board's auxiliary ADC (XADC) to measure the sensor temperature which is used in
conjunction with the calibration data to provide temperature corrected power
measurements.
This assembly produces the differential drive signal for the power sensor's
chopper. The processor assembly programs the level and frequency of these
signals:
– For 848x Series power sensors, this signal is 0 V and −10 V at 217 Hz.
– For E-Series power sensors, this signal is +7 V and −3 V at 434 Hz.
The XADC is also used to support system built in test by being capable of
digitizing a signal routed to it from the processor assembly.
As described earlier the E-Series power sensors have built in serial EEPROM and
range switching which is controlled or accessed via the measurement assembly.
The N-chop sensor wire used by the 848x Series power sensors doubles as a serial
clock and the sensor resistor line doubles as a bi-directional serial data line. The
wire labelled "Auto Zero" is also used as a serial bus enable control.
The auto zero line is grounded to the sensor ground line when an 848x Series
power sensor is used. Autozeroing is accomplished without the need for this
signal in the power meter.
All clocks are derived from the 20 MHz system clock that is generated on the A2
processor assembly and distributed on the A5 daughter assembly.
The E9320 sensors are provided with switchable power supplies.
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